State the visibility level for pipeline subscriptions

doc/ci/multi_project_pipelines.md

@@ -322,6 +322,9 @@ will now trigger a pipeline on the current project's default branch. The maximum
 number of upstream pipeline subscriptions is 2 by default, for both the upstream and
 downstream projects. This [application limit](../administration/instance_limits.md#number-of-cicd-subscriptions-to-a-project) can be changed on self-managed instances by a GitLab administrator.
 
+The upstream project needs to be [public](../public_access/public_access.md) for
+pipeline subscription to work.
+
 ## Downstream private projects confidentiality concern
 
 If you trigger a pipeline in a downstream private project, the name of the project